1. Field of the Invention
The present invention relates to an optical property measuring method and an optical property measuring apparatus for measuring an optical property of a colored surface on a fluorescent substrate.
2. Description of the Related Art
Today, paper and fabrics are often treated by FWA (Fluorescent Whitening Additives). It is extremely difficult to evaluate the color of a colored surface of these products or an article using these products as a substrate, without considering an effect of fluorescent light. In view of the above, there is a demand for improvement in colorimetry of a colored surface of the FWA treated paper or fabrics, or the article using these products as the substrate, considering an effect of fluorescent light.
Generally, a visible property of a reflecting sample is expressed relatively to the perfect white, namely, based on the total spectral radiance factor B(λ). The total spectral radiance factor B(λ) is the ratio of light emitted from the sample to that from the perfect reflecting diffuser in the identical illuminating and receiving conditions at each wavelength λ.
A color of fluoresced light is observed as a light source color alone. In case of a sample containing a fluorescent substance (hereinafter, called as “fluorescent sample”), however, the fluoresced light is added to the reflected light, and the color thereof is observed as an objective color. That is, the light emitted from the fluorescent sample is the sum of the reflected light and the fluoresced light, and accordingly, the total spectral radiance factor B(λ) of the fluorescent sample is also given as the sum of reflection spectral radiance factor R(λ) and the fluorescent spectral radiance factor F(λ) which are the ratios of light reflected and fluoresced from the sample respectively to the light from the perfect reflecting diffuser in the identical illuminating and receiving conditions as expressed by Equation (1).B(λ)=R(λ)+F(λ)  (1)
Since the above-mentioned perfect reflecting diffuser has no fluorescence, and the reflectivity thereof has no dependence on the wavelength of illumination light, the above-mentioned total spectral radiance factor B(λ), reflection spectral radiance factor R(λ) and fluorescent spectral radiance factor F(λ) are equivalent to the ratios of the light emitted, reflected and fluoresced from the sample respectively to the illumination light with a suitable proportional coefficient. An object of the colorimetry is to obtain a measurement value analogous to visual observation. The color of a fluorescent sample is observed as an objective color, and accordingly is related to the total spectral radiance factor B(λ), from which the colorimetric values are derived.
CIE (International Committee of Illumination) defines spectral intensity distributions of several standard illuminations for colorimetry such as Illuminant D65, D50, D75 (daylight), Illuminant A(incandescent lamp), Illuminant F11, and Illuminant C. For the evaluation of fluorescent samples, Illuminant D65 is generally used. The spectral excitation-fluorescence characteristics of a fluorescent sample or a fluorescent substrate illuminated with illumination light is expressed by the Bi-spectral Luminescent Radiance Factor (referred to as “BLRF” hereinafter) F(μ,λ) which is the matrix data showing the intensity of the fluoresced light at wavelength λ excited by excitation light i.e. incident light of wavelength μ for illuminating the fluorescent sample surface with a unit intensity i.e. by monochromatic light of a unit intensity at wavelength μ.
An example of the above-mentioned matrix data is shown in FIG. 8, wherein the cross-section along the fluorescence wavelength λ expresses the spectral excitation efficiency for fluorescing at wavelength λ while the cross-section along the excitation wavelength expresses the spectral intensity of fluoresced light excited at wavelength μ. Accordingly, a sample containing a fluorescent substance of the bi-spectral luminescent radiance factor F(μ,λ) has the fluorescent spectral radiance factor F(λ) expressed by Equation (2), where the proportional coefficient is neglected, when illuminated by illumination I of the spectral intensity I(λ).F(λ)=∫F(μ,μ)·I(μ)dμ/I(λ)  (2)
That is, F(λ) is obtained as the ratio of convolution of the spectral intensity I(μ) of illumination I and the bi-spectral luminescent radiance factor F(μ,λ) to I (λ).
As indicated by Equation (2), the fluorescent spectral radiance factor F(λ) depends on the spectral intensity I(μ) of the illumination I. Accordingly, the total spectral radiance factor B(λ), which is the sum of the reflection spectral radiance factor R(λ) which itself does not depend on the spectral intensity I(μ) of the illumination I and the fluorescent spectral radiance factor F(λ), and the colorimetric values derived therefrom also depend on I(μ).
As the result, the spectral intensity I(μ) of illumination light (referred to as “test illumination” hereinafter) need to be specified when evaluating the optical property of a fluorescent sample and for the accurate measurement, the spectral intensity I(μ) of illumination light of a measuring apparatus need to be the same as that of the specified test illumination. However, it is difficult and expensive to realize such an illumination of the same spectral intensity as that of the standard illuminant D65 or C generally used as the test illumination.
Alternatively, the total spectral radiance factor B(λ) or the fluorescent spectral radiance factor F(λ) can be calculated using Equation (2) with the measured bi-spectral luminescent radiance factor F(μ,λ) or bi-spectral radiance factor B(μ,λ) of the sample and the spectral intensity I(μ) of the test illumination given as numerical data. Here, similarly to the bi-spectral luminescent radiance factor F(μ,λ), the bi-spectral radiance factor B(μ,λ) is the matrix data showing the intensity of the total emission which is the sum of the fluoresced light at wavelength λ excited by monochromatic light of a unit intensity at wavelength μ and the reflected light. The total spectral radiance factor B(μ,λ) is obtained as the ratio of the convolution of the spectral intensity I(μ) of the illumination I and the bi-spectral radiance factor B(μ,λ) to I(λ).B(λ)=∫B(μ,λ)·I(μ)dμ/I(λ)  (2-1)
However, since the measurement of the bi-spectral luminescent radiance factor F(μ,λ) or the bi-spectral radiance factor B(μ,λ) requires a complicated and expensive bi-spectro-fluorometer e.g. a double monochromator comprising two spectral units, one for illumination and the other for receiving, and long time for measurement, this method is not practical. Quality controls of products treated by FWA such as paper are performed generally using either of the following two simplified methods.
(Gaertner and Griesser's Method)
As shown in FIG. 10, a fluorescent sample 601 is placed at a sample aperture 603 of an integrating sphere 602 of a measuring apparatus 600 for measuring an optical property. A light source 604 such as a xenon flash lamp contains a sufficient UV component, and a light flux 605 from the light source 604 passes through the aperture and enters into the integrating sphere 602. A UV cut filter 605 is inserted so as to partially block the optical path of the light flux 605, and a part of the light flux 605 which passes through the UV cut filter 606 has the UV component eliminated. The degree of insertion of the UV cut filter 606 is adjustable so as to allow adjustment of the UV intensity in the illumination light. The light flux 605 partly passing through the UV cut filter 606 and entering into the integrating sphere 602 undergoes diffuse reflection within the sphere 602 and forms diffuse light which illuminates the fluorescent sample 601. Radiant light 607 emitted in a predetermined direction from the illuminated surface passes through the observation aperture and enters a sample spectral unit 608 for detecting the spectral intensity Sx(λ). Similarly, a light flux 609 having substantially the same intensity as the illumination light of the fluorescent sample 601 enters a monitoring optical fiber 610 so as to be directed to a monitoring spectral unit 611 for detecting the spectral intensity Mx(λ). A controller 612 calculates the total spectral radiance factor Bx(λ) based on information on the spectral intensities Sx(λ) and Mx(λ) detected by the spectral units 608 and 611. (see FIG. 4 of Japanese Unexamined Patent Publication No. Hei 8-313349 corresponding to U.S. Pat. No. 5,636,015).
A fluorescent standard containing a fluorescent substance with excitation-fluorescence characteristics, namely, the bi-spectral luminescent radiance factor F(μ,λ) substantially identical or similar to that of the sample to be measured and given a colorimetric value such as CIE whiteness under the specific test illumination is used to determine the degree of insertion of the UV cut filter 606. The fluorescent standard is measured by the measuring apparatus 600, and the UV intensity is corrected by adjusting the degree of insertion of the UV cut filter 606 so as to match the value of CIE whiteness calculated from the obtained total spectral radiance factor Bx(λ) to the CIE whiteness given to the fluorescent standard.
Gaertner and Griesser's method is mechanically complicated and unreliable, and also requires complicated and time-consuming operation, that is, measurements and movements of the UV cut filter need to be repeated until the measured calorimetric value, CIE whiteness, for example, agrees the given value. This method results in the single specific colorimetric value, CIE value, in this case, compatible to that under a specific test illumination. However, in principle, the multiple calorimetric values, the CIE whiteness and Tint value, for example, or the total spectral radiance factor Bx(λ) are not compatible simultaneously.
(Method of U.S. Pat. No. 5,636,015)
As described above, Gaertner and Griesser's method modifies the UV content in the illumination first and modifies the total spectral radiance factor Bx(λ) as the result. This method numerically synthesizes the total spectral radiance factor Bx(λ) first and synthesizes the illumination of the spectral intensity necessary for Bx(λ) as the result. In U.S. Pat. No. 5,636,015, as shown in FIG. 11, an integrating sphere 702 of a measuring apparatus 700 is provided with a first illuminator 704 for emitting a light flux 703 containing a UV component and a second illuminator 706 for emitting a light flux 705 containing no UV component. The measuring apparatus 700 is further provided with a first spectral unit 709 for detecting the spectral intensity of emitted light 708 from a fluorescent sample 701 placed at a sample aperture 707 and a second spectral unit 712 for detecting the spectral intensity of a light flux 710 of the illumination through the optical fiber 711, and a control unit 713.
In the measuring apparatus 700, the fluorescent sample 701 is illuminated by the first and second illuminators 704 and 706 consecutively, and the spectral intensities Sx1(λ) and Sx2(λ) of emitted light from the sample, and the spectral intensities Mx1(λ) and Mx2(λ) of the illumination light are respectively detected. The total spectral radiance factors Bx1(λ) and Bx2(λ) corresponding to the illuminations by the first and second illuminators 704 and 706 are obtained from Sx1(λ), Sx2(λ), Mx1(λ), and Mx2(λ) and thus, the total spectral radiance factor Bxc(λ) is synthesized by linearly combining Bx1(λ) and Bx2(λ) with the weighting factor W(λ) as shown in Equation (3).Bxc(λ)=W(λ)·Bx1(λ)+(1−W(λ))·Bx2(λ)  (3)
Similarly to Gaertner and Griesser's method, the above-mentioned weighting factor W(λ) for each wavelength λ is determined using a fluorescent standard containing a fluorescent substance with excitation-fluorescence characteristics, namely, the bi-spectral luminescent radiance factor F(μ,λ) substantially identical or similar to that of the sample to be measured and given a total spectral radiance factors Bs(λ) under the specified test illumination. That is, the weighting factor W(λ) is so determined for each wavelength λ numerically that the synthesized total spectral radiance factor Bxc(λ) by Equation (3) matches the given total spectral radiance factors Bs(λ) under the specific test illumination. (see FIG. 1 of U.S. Pat. No. 5,636,015).
This method is equivalent to respectively performing the correction of the UV content in the illumination by Gaertner and Griesser's method for the total spectral radiance factor Bx(λ) at each wavelength as the target instead of the single colorimetric value. Since the method gives the total spectral radiance factors Bxc(λ) of the sample comparable to Bs(λ) under the specific test illumination, the method has an advantage that all the colorimetric values derived therefrom are also comparable to those under the specific illumination. Although this method eliminates many shortcomings of Gaertner and Griesser's method such as the mechanical complicity, lack of reliability, and complicated and time-consuming operation, it still requires a fluorescent standard, and errors due to the difference between the spectral intensity of the illumination at the time of UV correction and that at the time of sample measurement thereafter still remains.
If paper is treated by FWA, colors printed thereon are affected by fluorescence of the paper i.e. a fluorescent substrate. Since the amount of excitation light reaching the paper depends on the spectral transmittance of ink covering the paper, the spectral excitation-fluorescence characteristics (spectral excitation efficiency and spectral fluorescence intensity) of the printed paper depend not only on the spectral excitation-fluorescence characteristics of the paper but also on an average spectral transmittance characteristic of a measuring area of paper including dot areas. The average spectral transmittance characteristic of the measuring area depends on a spectral transmittance characteristic of a whole printed surface printed with an ink at an area ratio of 100%, and a ratio of the dot area (relative area covered by ink) with respect to the measuring area of paper. Consequently, the spectral excitation-fluorescence characteristics of the measuring area depend on the spectral transmittance characteristic of the whole printed surface, and the ratio of the dot areas with respect to the measuring area. If paper is printed with two or more different inks, the paper is covered with the inks and the superposition of the inks, and accordingly, the spectral excitation-fluorescence characteristics of the measuring area depends on the spectral transmittances of the whole printed surfaces of the inks and the superposition of inks, and the area ratios of the dot areas. In other words, information on spectral transmittance characteristics of whole printed surfaces where inks are individually and superimposedly printed is required in order to measure an optical property of a printed surface on paper i.e. a fluorescent substrate where two or more different inks are printed, considering an effect of fluorescent light.
Ink with a transmittance characteristic having no dependence on the wavelength does not change the relative spectral intensity of the illumination light reaching the paper and equally influences to the spectral integrated excitation efficiency of the illumination synthesized by the method of U.S. Pat. No. 5,636,015 and to that of the test illumination. Accordingly, the synthesized total spectral radiance factor Bxc(λ) of the printed surface is comparable to that to be obtained under the specified test illumination although it is different from that of an unprinted surface. Here, the spectral integrated excitation efficiency E(λ) expressed by Equation (4) is the excitation efficiency for fluorescence at wavelength λ excited by the whole illumination.E(λ)=∫Q(μ,λ)·I(μ)dμ  (4)where Q(μ,λ) is the bi-spectral excitation efficiency, that is, the excitation efficiency for fluorescence at wavelength λ excited by light of a unit intensity and of bandwidth dμ at wavelength μ.
As described above, both simplified methods (Gaertner and Griesser's method and the method of U.S. Pat. No. 5,636,015) need the fluorescent standard. Since the fluorescent standard made of the same material as the sample to be measured such as paper or fabric and containing the same fluorescent substance as that contained in the sample is unstable and requires considerable cares for controlling the change due to the aging and for the renewal. Further, errors due to the change of the spectral intensity of the illumination after the UV correction are inevitable, and as the result, frequent UV readjustments are required for avoiding these errors. In view of these, a method and an apparatus for measuring a fluorescent sample free from a fluorescent standard and a UV correction using the fluorescent standard are required. In order to accomplish the task, it is required to accurately acquire spectral transmittance characteristics of inks, in addition to an excitation-fluorescence characteristics of printed paper, and area ratios of the inks. However, the ink concentration differs among lots. Also, the thickness of each ink layer differs depending on a printing machine e.g. a printer, an ambient temperature, or a like factor. Accordingly, if a spectral transmittance characteristic different from a spectral transmittance characteristic of a whole printed surface printed with an ink at an area ratio of 100% under a specific printing condition such as a printing machine, an ink lot, an ambient temperature, or a like factor, in other words, a spectral transmittance characteristic of a whole printed surface in a different printing condition, despite the same ink area ratio of 100%, is used as a typical value, such an approach may cause a non-negligible measurement error.